AutoCAD: Attachments or Overlays?

Know what it means to use Attachments or Overlays as XREFs. It is important to avoid file troubles.
Català - Castellano
XREFs can be inserted as Overlays or as Attachments. That choice will affect the behaviour of the drawing where they are inserted.
  • Overlay: Inserting a drawing A as an overlay in drawing B is the equivalent of just allowing that external reference A to bee seen in the current drawing B. The external reference does not become part of that drawing. When inserting drawing B in another drawing, let's say C, A will not appear.
  • Attachment: Attachments work the opposite way. An external reference inserted as an attachment becomes part of the drawing where it is inserted. In this case, drawing A would become part of Drawing B when inserted as an attachment. They would still remain two separate drawings, but any time we'd attach drawing B in lets say drawing C, drawing A would be attached too since it is "part" of drawing B.
Letters apart, consider the implications of this. If you are working on a section and insert a floor plan as an attachment, this floor plan will become part of the section drawing. Then if you work on the floor plan and insert the section (as attachment or overlay, it doesn't matter) you will be also inserting the floor plan (the same you are working on) creating a circular reference.
You will get a warning message when this happens, and remember, when you do is it time to consider what are yo doing. Circular references are number 1 ingredient i a recipee for disaster.
In general, when projects get more and more complicated, the amount of circular references could be so big, you will have big trouble detaching XREFs or worse you will be unable to batch Plot, etransmit files, etc.
In my team we have long ago decided to not use attachments at all, since they have been giving us a lot of trouble.
The use of attachments is in general only justified to save some steps reattaching drawings. On certain situations, you might choose to use them, sometimes it feels almost unavoidable, but remember the possible problems described when doing so and consider if an overlay would do the trick.
My advice is to use always overlays, only use attachments wisely and because you feel there is no other option. And s a general rule, if you are in charge of the file structure of a project, don't let anyone else use attachments except with your permission. Knowing who is using attachments and why will allow you to control where the problems could arouse and how to solve them.

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